In the field of liquid crystal display devices, a backlight including a side edge-type (also referred to as side light type) light guide plate that outputs light from the light source in a planar manner using a light guide plate is often used in order to obtain a thinner device. In addition, a technique is known in which area control is conducted by separating a light guide plate into a plurality of regions with a light source provided for each separated region, and emitting light only from a separated region (light-emitting region) corresponding to a desired light source by turning on this light source. Through such area control, it is possible to improve display quality by attaining pseudo-impulse-type display or adjusting the brightness of specific areas of the backlight while controlling the light transmittance of the liquid crystal layer in the liquid crystal display element.
Patent Document 1 discloses an illumination device in which area control of the light guide plate is attained, for example. FIG. 16 is a plan view of the illumination device of Patent Document 1. FIG. 17 is a cross-sectional view along the line A-A′ of FIG. 16, viewed in the direction of the arrows. An illumination device 101 in FIGS. 16 and 17 is provided with a light guide plate 102 constituted of a plurality of blocks 102a aligned in a row, and a plurality of light sources 103, which are each provided for each block 102a, and which emit light to the blocks 102a. In at least some of the regions between adjacent blocks 102a, a gap of at least 0.1 microns, which is an air layer 105, is formed, which optically separates the blocks from each other. Also, at least one of the upper surface and the lower surface of the light guide plate 102 has formed thereon a prism or a pattern (not shown in drawings) to diffuse light.
If light is emitted from one light source 103 in the illumination device 101 of FIGS. 16 and 17, then this light enters one block 102a from an edge face of the light guide plate 102, and the light undergoes total reflection from surfaces of the block 102a (in other words, faces 102b that are side faces of the light guide plate 102 or boundaries between the blocks 102a and the air layers 105, and the upper surface and the lower surface of the block 102a), and is transmitted through the block 102a in a direction away from the light source 103. In FIG. 16, a light path L is shown as an example of the transmission path of this light. During transmission, light that is scattered by the prism or pattern formed on the upper surface and/or the lower surface of the block light 102a, and thus, no longer satisfies the total reflection condition, is outputted towards the outside of the block 102a from the upper surface or the lower surface. Light is evenly outputted from the light guide plate 102 by providing the prism or pattern on the block 102a with an increased density in positions further downstream along the direction that the light travels. As a result, planar light is emitted from the light guide plate 102. Thus, when emitting light from one block, other blocks adjacent thereto do not emit light, which allows pseudo-impulse display to be attained.
Patent Document 1 also discloses a light guide plate shown in FIG. 18. FIG. 18 is a configuration corresponding to that of FIG. 17. In FIG. 18, wedge-shaped grooves 284c are formed in a light guide plate 282c, and the grooves 284c separate the light guide plate 282c into a plurality of blocks 282d. Thus, the shape of the blocks 282d in a cross-section that intersects perpendicularly with the lengthwise direction of the blocks 282d is trapezoidal. The taper angle of the block 282d, or in other words, the angle of the side face of each groove 284c with respect to a direction perpendicular to the surface of the light guide plate 282c is 5°, for example. Also, the grooves 284c penetrate the light guide plate 282c in the lengthwise direction thereof, but do not penetrate the light guide plate 282c in the thickness direction thereof. As a result, adjacent blocks 282d are optically joined in some portions.